The present invention relates in general to an orthopaedic hip implant, and in particular to a modular orthopaedic hip implant that can be custom fit to a patient. The present invention also relates to an implant having a shock absorption system which increases patient comfort and the life of the implant.
Prosthetic implants for the replacement of a portion of a patient""s hip joints are well-known, and are typically available as a two to three component system. The femoral stem component includes a shank at its distal end which extends into the medullary canal of the femur and is fixed therein using bone cement or other means known in the art. At the proximal end of the femoral stem is a neck which typically terminates in a spherical ball that is adapted to cooperate with the patient""s acetabulum or a prosthetic acetabular cup fixed into the patient""s acetabulum. The ball, neck and femoral stem are typically formed in one piece from cobalt-chromium-molybdenum, titanium or other suitable material. The acetabular cup is typically formed as a metal hemispherical bowl and, optionally, can be provided with a plastic insert that is fixed therein to enhance the sliding engagement between the ball and the acetabular cup.
Conventional one-piece femoral component designs are available in different sizes, but they do not allow enough flexibility for variations in individual parameters of the patient""s anatomy. Parameters such as femoral neck length, femoral shank length and diameter, and femoral head size can vary independently of one another. For example, the length of the femoral neck can vary independently of the size of the femoral head or length of the medullary canal. It can thus be difficult to find a suitably fitting implant when selecting the implant from the different sizes available as one-piece femoral components.
To address this problem, modular implant systems are known. For example, U.S. Pat. No. 4,938,773 to Strand discloses a femoral stem which can be fitted with interchangeable, different size femoral neck portions. Such a system is undesirably limited by the availability of different size components. Further, a large quantity of different size components must be produced and stocked to ensure that all patients can be fitted.
Similarly, U.S. Pat. No. 5,507,830 to DeMane discloses a modular hip prosthesis which includes a plurality of removable, different size tubular sleeves that can be attached to a cylindrically shaped stem of the femoral component, thereby allowing the surgeon to extend the stem length as necessary. Also disclosed are interchangeable sleeves that can be added to the neck portion of the implant to elongate the neck portion of the prosthesis. Removable pads are provided for attachment to the mid-section of the prosthesis for changing the cross-sectional configuration thereof. Again, such a system is limited by the availability of different size sleeves or extensions.
Another problem with conventional hip implants is that countless compressive stresses are transmitted thereto from daily activities such as walking, running, exercising, sitting and standing. These compressive stresses can eventually cause painful fractures and can often result in the implant loosening after several years. Ultimately, revision surgery may become necessary.
Prosthetic hip implants that address impact problems are known in the art. For example, SU 1718883-A1 discloses a modular implant that includes a spring disposed at the end face of the femoral neck, the threaded end of which is screwed into the base of the femoral component. The spring is rigidly mounted in the bottom of a recess formed in the prosthetic femoral head and provides shock absorption for the implant.
U.S. Pat. No. 5,389,107 to Nassar et al. discloses a prosthetic hip implant having an elongate element that extends coaxially from the ball section of the femur component. The elongate element slidably extends into a chamber formed by a tubular insert that is secured in the femur. Contained at the bottom of the chamber is a spring against which the elongate element abuts, thereby providing shock absorption. A pin member extends from the bottom of the chamber and slidably fits into a bore formed in the elongate element. A second spring is disposed between the pin and the bottom of the bore to provide further shock absorption.
What is needed is an improved modular implant that also provides shock absorption.
The present invention provides a modular hip implant that can be custom fit to an individual patient and that includes a shock absorption system that absorbs compressive stresses that are imparted to the implant.
In one form thereof, the present invention provides a modular hip prosthesis. The hip prosthesis comprises a ball member having an outer surface adapted to cooperate with an acetabular socket and a femoral stem having a shank adapted to be inserted and secured into a medullary cavity of a femur. The femoral stem has a neck at a proximal end thereof which is connected to the ball member. A spring mechanism is disposed intermediate the ball member and the neck, and provides cushioning movement between the femoral stem and the ball member. The spring mechanism is detachably connected to the neck and detachably connected to the ball member.
In a preferred form, the modular hip prosthesis further comprises a bore disposed in the ball member. A coupling member houses the spring mechanism, and the connection of the ball member to the spring mechanism is through the coupling member. The coupling member is received in the bore to an adjustable depth, adjustment of which causes corresponding adjustment of the distance the neck extends from the ball member. More preferably, the bore and the coupling member comprise corresponding threads, the coupling member being threadingly received in the bore. Still more preferably, the spring mechanism includes a first connector at a first end thereof connecting the neck to the spring mechanism. The first connector and the neck include complementary threads, such that the first connector is threadingly connected to the neck. The spring mechanism includes a second connector at a second end thereof connecting the spring mechanism to the coupling member.
In another form thereof, the present invention provides a method of custom fitting a hip prosthesis to an individual patient. In this method, a ball member is selected from a plurality of different size ball members, depending upon the size of the acetabular socket into which the ball member is to be inserted. A femoral component is selected from a plurality of different size femoral components, and the neck of the selected femoral component is attached to a coupling member. A depth that the coupling member is to be inserted into the selected ball member is determined. Such depth corresponds to an individual patient. The coupling member is installed into the ball member to the determined depth.
In a preferred form of the inventive method, a spring mechanism is installed in the prosthesis to allow cushioning movement of the neck of the selected femoral component relative to the ball member. More preferably, the spring mechanism is selected from a plurality of spring mechanisms having spring elements of different spring constants or stiffnesses. The spring stiffness can be calibrated to the weight of the patient. Further, the length of the neck can be adjusted intraoperatively to compensate for errors in neck length obtained from preoperative imaging techniques.
One advantage of the present invention is that the spring mechanism absorbs much of the compressive stresses imparted to the implant during daily activities such as walking, running and exercising. Because the spring mechanism contracts and expands to absorb load bearing, shock and compressive stresses imparted to the hip joint during weight bearing and mobilization, the implant is less likely to loosen, and the useful life of the implant is therefore lengthened. The spring mechanism also reduces other complications, such as dislocation of the femoral stem from the acetabulumn, acetabular damage and erosion, and protrusion of the femoral ball member into the acetabulum and pelvis during a sudden jarring event, such as a fall.
Another advantage of the present invention is that the spring mechanism is a modular component such that a spring element having a specific stiffness can be selected.
Another advantage of the present invention is that the length of the femoral neck can be changed without adding or interchanging parts, unlike the above-described prior art implants which require a plurality of interchangeable parts. Instead, the present invention employs a single coupling member that can be installed in the ball member to a depth which corresponds to the desired length of the femoral neck.
Yet another advantage of the present invention is that the length of the femoral neck can be adjusted intraoperatively. While pre-operative imaging techniques can be used to determine the appropriate length of the femoral neck, such techniques are often only an approximation of actual surgical conditions. With the present invention, adjustments to the length of the femoral neck can be made during surgery by adjusting the depth to which the coupling member is inserted into the ball member so that an exact preoperative neck length need not be entirely relied upon.